The invention is directed to injection molding of oral brush bodies.
Injection molding is commonly used in the manufacture of oral brush bodies. Injection molding processes often involve injecting a thermoplastic polymer into a mold cavity under hydraulic pressure, allowing the polymer to harden, opening the mold, and then removing the molded body from the mold cavity. The mold cavity is defined by two mold halves. A clamping unit maintains the mold halves in a closed position while the polymer is injected into the cavity. The clamping unit applies a hydraulic pressure to close the mold and to maintain the mold halves in closed position until the mold cycle is complete.
The injection unit of an injection molding apparatus for simultaneously molding multiple oral brush bodies often experiences greater than 1000 psi of pressure during the process of injecting polymer into the mold cavity. This same apparatus often also has an idle pressure, i.e., the pressure of the molding apparatus during the period in which polymer is not being injected into the mold cavity, of at least 200 psi. The high pressures applied to the molding apparatus during a mold cycle can cause wear on the many components of the molding apparatus including, e.g., the clamping unit.
In one aspect, the invention features a method of making an oral brush body using an injection molding apparatus that includes a mold cavity defining at least a portion of the oral brush body. The method includes injecting polymer into the mold cavity at a pressure no greater than about 250 psi and in an amount sufficient to fill the cavity. In one embodiment, the pressure is from about 100 psi to about 250 psi. In another embodiment, the pressure is from about 130 to about 200 psi. In other embodiments, the pressure is no greater than about 200 psi. In one embodiment, the method further includes altering the pressure during an injection molding cycle from the first pressure to a second pressure, the second pressure being no greater than about 220 psi.
In some embodiments, the method further includes injecting the polymer into a portion of the cavity at a pressure no greater than 850 psi prior to injecting the polymer at a pressure of no greater than about 250 psi, and reducing the pressure to no greater than about 250 psi. In another embodiment, the method further includes removing the oral brush body from the mold cavity. In some embodiments, the mold includes a plurality of mold cavities each defining an oral brush body, and the injecting further includes injecting polymer into the plurality of mold cavities at a pressure no greater than about 250 psi and in an amount sufficient to fill the cavities.
In one embodiment, the mold includes 22+n cavities each defining an oral brush body where n=0-3.
In other aspects, the invention features an injection molding apparatus that includes a mold that includes a cavity defining at least a portion of an oral brush body, and an injection unit in communication with the cavity, the injection unit being capable of injecting a polymer into the mold cavity at a pressure no greater than about 250 psi and in an amount sufficient to fill the cavity. In one embodiment, the apparatus further includes a control in communication with the injection unit, the control being capable of changing the pressure exerted on the injection unit during the course of an injection molding cycle. In other embodiments, the apparatus further includes a control in communication with the injection unit, the control being capable of comparing the amount of pressure exerted on the injection unit during a first injection molding cycle with a predetermined pressure and automatically adjusting such that the pressure exerted during a subsequent molding cycle achieves the predetermined pressure.
In another embodiment, the mold includes a plurality of cavities, each cavity defining at least a portion of an oral brush body, the injection unit being capable of injecting polymer into the cavities at a pressure no greater than about 250 psi and in an amount sufficient to fill the cavities. In one embodiment, the polymer includes polypropylene. In other embodiments, the polymer has a melt flow rate of from about 12 gm/10 min to about 35 gm/10 min.
In another aspect, the invention features an injection molding apparatus that includes: a) a mold that includes a plurality of cavities defining an oral brush body, b) an injection unit in communication with the cavities, and c) a control capable of applying a pressure of no greater than about 200 psi on the injection unit.
The injection molding process and apparatus are useful for manufacturing oral brush bodies and oral brushes at low injection pressures, i.e., pressures no greater than about 250 psi. The apparatus is capable of an idle pressure of 0 psi and can inject polymer into a mold cavity at pressures less than 250 psi. The low pressure of the process imparts less wear on the mold machinery relative to high pressure molding thereby increasing the useful life of the mold machinery.
The injection molding apparatus can also alter the amount of pressure exerted on the ram of the injection unit, and thus the polymer being injected, a number of times (e.g., 1-20 times) during the injection molding cycle. The ability to change the pressure throughout the molding cycle provides greater control over the injection molding process and, in particular, the stages of the injection molding cycle that involve filling more detailed and intricate parts of the body such as, e.g., a logo.
Polymer compositions having low melt flow indices, e.g., low viscosity polypropylene, can be used in the injection molding apparatus. The injection molding apparatus can inject polymer compositions into mold cavities or mold inserts that include individual tufts of bristles extending into the cavity at a predetermined position, e.g., to a predetermined depth, at a predetermined orientation, in a predetermined arrangement and combinations thereof The low pressure of the process enables the molten polymer to surround the exposed portion of the tuft, e.g., the tuft ball, and encapsulate the tuft ball, without altering the predetermined position of the tufts to such an extent that the resulting brushes fail to meet a predetermined criteria. The apparatus provides processes that simultaneously mold and tuft of oral brushes.
Additional features and advantages of the invention will be apparent from the description of the preferred embodiment thereof and from the claims.